As the capacitor discharges, it drives the motor in reverse as it did when the antenna was bumped. This pulls down the base of PNP transistor Q2 (connected in parallel with the antenna switch) and charges the capacitor. The electret-microphone’s signal is amplified by two transistors to create a negative going pulse. It takes approximately two seconds for the capacitor to finish discharging, and then the bridge goes back to the motor-forward state.Ī loud sound triggers the same circuit. With the collector low, the H-bridge is now driven to the motor-reverse state. When the antenna is bumped, it charges the 47uf capacitor C1, which discharges through a 6.8k resistor and the base of NPN transistor Q1 causing the collector to go low. The collector of NPN transistor Q1 is normally high, keeping the H-bridge in the motor-forward state.
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